On page 197 of V. (Harper Perennial edition) a witch-like woman tells Ferrante what he already knows: that there is a network of natural tunnels beneath the earth. She goes on to say "'...Tell me what Vheissu is really the code name for. Tell me, you idiot, what I already know: that it stands for Vesuvius.'"
Vesuvius is a famous volcano near Naples, Italy, and is site of the door to the underworld according to Virgil. The implication is that an entrance to a world-wide network of natural tunnels is at Vheissu. Several powers want access to these subterranean passages and to stop the strange race of people Old Stencil encountered at Vheissu from employing the tunnels for their own purposes.